Almost all uses actually only care about ADT substs, so it's better to
be explicit. The methods were a bad abstraction anyway since they
already didn't include the inner types of e.g. `TyKind::Ref` anymore.
- don't shift in/out for Chalk mapping (we want to have the same
binders now)
- do shift in when creating the signature for a closure (though it
shouldn't matter much)
- do shift in when lowering a `fn()` type
- correctly deal with the implied binder in TypeWalk
